.box-list {
  width: 200px;
}

.box-list .box {
  width: 100%;
  height: 100px;
  margin-bottom: 10px;
  background-color: blue;
}

.box-list .box:nth-child(3n) {
  background-color: blue;
}

.box-list .box:nth-child(3n + 1) {
  background-color: red;
}

.box-list .box:nth-child(3n + 2) {
  background-color: green;
}
